This invention relates to the conversion of paraffins into more valuable olefinic and aromatic products. In particular, the invention relates to a process for increasing the yield of valuable C.sub.2 -C.sub.4 olefins from the conversion of a stream rich in C.sub.5 -C.sub.9 paraffins, cycloparaffins and olefins in the presence of a medium-pore zeolite catalyst having a relatively low acid activity.
Conversion of paraffinic feedstocks to more valuable aromatic and olefinic product streams is known. For example, U.S. Pat. No. 3,756,942 discloses a process for the preparation of aromatic compounds in high yields which involves contacting a particular feed consisting essentially of mixtures of paraffins and/or olefins, and/or naphthenes with a crystalline aluminosilicate, e.g. ZSM-5, under conditions of temperature and space velocity such that a significant portion of the feed is converted directly into aromatic compounds. U.S. Pat. No. 3,759,821 discloses a similar process for upgrading catalytically cracked gasoline. Finally, U.S. Pat. No. 3,760,024 teaches a process for the preparation of aromatic compounds which involves contacting a feed consisting essentially of C.sub.2 -C.sub.4 paraffins and/or olefins with a crystalline aluminosilicate, e.g. ZSM-5. The above references are incorporated herein by reference and are cited in particular for descriptions of useful feedstocks and process conditions.
Conversion has been found to be enhanced in fluidized bed processes by maintaining the fluidized bed reaction zone in a turbulent regime. For example, U.S. Pat. No. 4,547,616 to Avidan et al. teaches a process for the conversion of oxygenates to lower olefins in a turbulent fluidized bed of catalyst. U.S. Pat. No. 4,746,762 to Avidan et al. teaches a process for upgrading light olefins in a turbulent fluidized catalyst bed reactor. The Avidan et al. references are incorporated herein by reference and are cited in particular for the details of fluidized-bed operating variables.
The paraffin conversion processes mentioned above evolve product slates containing varying amounts of desired olefins and aromatics. The most economically attractive of these compounds are light olefins such as ethylene, propylene, and butylene, C.sub.5 + gasoline rich in aromatics and hydrogen. Previous processes have sought to maximize C.sub.5 + aromatics yield. Recent strengthening in the light olefin market has focused attention on increasing light olefins yield, particularly in petrochemical plants where light olefins are a basic building block for a broad spectrum of end products including thermoplastics. By way of contrast, C.sub.1 -C.sub.3 paraffins (i.e. methane, ethane and propane) particularly in admixture, are less valuable chemicals which are generally used for fuel.
From the foregoing, it can therefore well be seen that it would be highly desirable to shift selectivity in a process for upgrading paraffinic feedstreams toward more valuable products including C.sub.6 -C.sub.8 aromatics and C.sub.2 -C.sub.4 olefins. Further, it would be particularly beneficial to react a relatively low value stream such as propane to enhance light olefin production in a paraffin upgrading process. Previous processes have employed supplemental feedstreams to control product selectivity.
The addition of selected supplemental feedstreams has been shown to effect desirable shifts in product selectivities. For example, European Patent Application EP 215-579A teaches a process in which the aromatics selectivity of an ethylene aromatization process is enhanced by the addition of at least 5 wt. % methane to the feedstock.
U.S. Pat. No. 4,565,897 to Gane et al discloses a process for producing aromatics from a feedstock comprising C.sub.3 /C.sub.4 hydrocarbons mixed with ethane. The feedstock preferably contains at least 70 wt. % of C.sub.3 /C.sub.4 hydrocarbons. The presence of ethane in the mixed feed was found to improve conversion selectivity for aromatics.
U.S. Pat. No. 4,754,100 to Sorensen et al described a process for the production of butanes and C.sub.5 + aliphatic hydrocarbons from propane in which unexpectedly high selectivity to n-butane, isobutane, pentanes and C.sub.6 + alipahtics is attained by the addition of a controlled amount of mono-olefin to the catalytic reaction zone.
The previous processing schemes for upgrading aliphatic, particularly paraffinic feedstreams have not addressed methods for increasing selectivity both to aromatics and to light olefins.
Propane, a naturally occurring material and a by-product of many petroleum refining processes, has a low economic value and is often burned as a fuel gas or used as a component in liquefied petroleum gas (LPG). Thus, a process which improves the selectivity or a paraffin upgrading process to yield more valuable products by co-feeding a relatively low value feedstock such as propane would provide a significant economic benefit.